2 endergonic exergonic All organisms, prokaryotic or eukaryotic, need to build the molecules they need, and find the energy to do so! Metabolic Pathways Most biochemical reactions are part of a series of reactions referred to as a metabolic pathway: it usu. takes multiple reactions to make end-product pathways can be catabolic or anabolic each reaction is catalyzed by its own enzyme Enzyme Basics Almost all biochemical reactions are catalyzed by a specific enzyme: proteins that accelerate the rate of a reaction without being changed themselves lower the activation energy (E a ) the need for enzymes provides a way to control or regulate biochemical reactions reactions won t occur unless the enzyme that catalyzes the reaction is present & active 2

3 Enzymes lower the Activation Energy **reactions won t occur unless the E a requirement is met** Enzymes physically bind Substrates Control of Enzyme Activity Biochemical reactions can be controlled by changes in enzyme activity, which can be influenced in several ways: 1) Changes in the amount of enzyme or substrate more enzyme &/or more substrate = more product! 2) Changes in temperature, ph or [salt] can effect enzyme structure, hence its activity 3) Availability of any necessary cofactors some enzymes don t work w/o a non-protein cofactor 4) Effect of inhibitors molecules that bind to enzymes & reduce their activity 3

4 Factors effecting Enzyme Activity Temperature reactions occur more rapidly as temperature rises **as long as enzyme is active (heat can denature enzymes) ph enzyme structure depends on ph **ph affects charge of R groups, protein structure [Substrate] reactions occur more rapidly as [substrate] rises **saturation occurs when [substrate] is high enough Enzyme Denaturation enzymes are polypeptides that retain their ability to function only when folded properly changes in temperature, ph or [salt] can disrupt amino acid R group interactions causing the protein to unfold, i.e. become denatured **mutations can also lead to misfolded, non-functional enzymes** Some Enzymes Require Cofactors can be a metal ion, vitamin, or other non-protein if the cofactor is organic, it is called a coenzyme enzyme is inactive w/o cofactor 4

6 Adenosine Triphosphate (ATP) Preferred source of useable energy for ALL cells: breaking bond of 3 rd phosphate releases ideal amt of energy bond is easily broken (low E a ) **This is why organisms convert food energy to ATP energy** How is ATP produced? In most organisms, energy from a food source is converted to energy in ATP by glycolysis followed by 1 of 2 processes: FERMENTATION (low ATP yield) or RESPIRATION (high ATP yield) Glycolysis Don t memorize this!! Glycolysis is a catabolic pathway by which sugars such as glucose (& several other food sources) are broken down to two 3-Carbon molecules of pyruvic acid (or pyruvate): releases energy to yield 2 ATP per glucose also transfers high energy electrons (+ H) to NAD + to yield 2 NADH 6

7 Oxidation/Reduction Much of the energy in food molecules such as glucose is captured as high energy electrons (e - ) by electron carriers such as NADH & FADH 2 when a molecule receives or gains electrons it is said to be reduced **e - are typically transferred as part of a Hydrogen atom** NAD+ NADH a molecule that gives up electrons (i.e., loses H) is said to be oxidized Fermentation ATP production begins & ends with glycolysis in organisms that ferment. Fermentation is all about recycling NAD + so that glycolysis can continue: glycolysis NAD + NADH fermentation NADH is oxidized to NAD + by reducing pyruvate to lactic acid for example * Different Fermentation Products Different organisms recycle NAD + in different ways, resulting in a variety of fermentation end-products. 7

8 Respiration After glycolysis, energy in pyruvate & NADH is used to produce much more ATP by respiration: KREBS CYCLE breaks down pyruvate to 3 CO 2, energy captured as e - by NADH & FADH 2 ELECTRON TRANSPORT e - from NADH, FADH 2 used to produce H + gradient CHEMIOSMOSIS H + gradient used to make ATP The Krebs cycle a cyclical metabolic pathway catalyzed by enzymes in the matrix of mitochondria requires 2-C acetyl groups connected to coenzyme A (acetyl-coa) (3-C) pyruvate + CoA (2-C) acetyl-coa + CO 2 (Krebs cycle) Electron Transport & Chemiosmosis Occurs in the mitochondria of eukaryotes and at the plasma membrane of prokaryotes. oxygen (O 2 ) is usually the final electron acceptor, but other molecules can play this role in anaerobic respiration 8

9 Lipid & Protein Catabolism Lipids and proteins can also be used as sources of energy to produce ATP different amino acids enter glycolysis or the Krebs cycle at various stages fatty acids are broken down to acetyl groups & fed into the Krebs cycle Summary of ATP Production Obligate anaerobes: fermentation or anaerobic respiration Obligate aerobes: aerobic respiration (& brief periods of fermentation) Facultative anaerobes: can survive via aerobic respiration OR fermentation 3. Autotrophic Processes 9

10 All organisms depend on Autotrophs Autotrophs can produce organic molecules from CO 2, an inorganic carbon source. all heterotrophs require an organic source of carbon organic molecules, directly or indirectly, come from autotrophs The source of energy for autotrophic processes can be: LIGHT: photoautotrophs that carry out photosynthesis CHEMICAL: chemoautotrophs that use various molecules as a source of high energy e - Light Reactions of Photosynthesis Electrons (from H 2 O) energized by sunlight: fuel the synthesis of ATP through electron transport & chemiosmosis (much like respiration) ultimately reduce NADP + to NADPH ATP & NADPH provide energy to fuel production of sugars in the dark reactions Dark Reactions Involves an anabolic pathway known as the Calvin-Benson cycle: endergonic reactions of this pathway are fueled by ATP & NADPH from the light reactions Don t memorize this!! resulting sugars can be used as a source of energy or to build other organic molecules 10

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